1. Field of the Invention
The present invention relates to a signal reception method and a signal reception apparatus compliant with the CDMA (code division multiple access) communication scheme.
2. Description of the Background Arts
In a CDMA communication system, a sending end spreads transmission data over a wideband spectrum range using a predetermined individual spreading code assigned in advance, and the receiving end despreads (i.e., demodulates) the reception signal through the use of the identical spreading code. As a spreading code, a pseudo-random code sequence, for example, has been typically used, and if the synchronization in the spreading codes is not established between the sending and receiving ends, the despreading will not be properly achieved.
Generally, the radio propagation route between a sending end and a receiving end is not necessarily single, and a plurality of the propagation routes can be present due to reflection and/or diffraction of the radiowave. The receiving end receives the signals that have propagated through the plurality of the propagation routes, at different timings. The propagation route is commonly called a path. The CDMA communication system individually despreads the plurality of reception signals that have arrived through different paths and thus in different propagation times with different reception timings and then combines the despread signals, thereby achieving the diversity effect and improving the reception sensitivity and reception quality. This manner of combining the despread signals resembles gathering of several things by a rake. For this reason, the combination is called a RAKE combination. In addition, the arrangements for individually effecting the despread on a reception-signal basis are called fingers. The receiver adapted for the RAKE combination is provided with a plurality of fingers and capable of despreading the reception signals as many as up to the number of the fingers to achieve a RAKE combination.
The receiver in the CDMA communication system, i.e., the CDMA receiver, performs path search to detect the timing synchronization of the spreading codes on a path-by-path basis. A delay profiles is generated through the path search. In the path search, the correlation level at each timing between the spreading code of the receiving end and the reception signal is determined by computing the correlation between the former and latter while shifting the timing (the synchronization time) of the former in small steps. The correlation level thus calculated is called a reception level. Representing the timing shift in the abscissa and the correlation level in the ordinate over one signal-reception period makes up a delay profile. The delay profile represents the correlation power distribution of the signal of interest against a shift of timing, i.e., a delay time. In a delay profile, each of the peaks, which represent high correlation levels, corresponds to a reception path. Accordingly, by despreading the reception signal in synchronization with the timing of each peak, normal and high-level demodulation of the reception signal can be achieved. Therefore, the target of performing the path search resides in the establishment of the synchronization timing of the spreading code for each of the fingers.
A case may happen, however, that the state of propagation between a sending end and a receiving end varies from time to time or that, when at least one of the sending and receiving ends moves, for example, a new path suddenly comes out or the path of a high reception level, which has so far existed, is suddenly lost, as the moving end moves. For this reason, it is required constantly to perform the path search to update the delay profile and further to update the synchronization timing for each of the fingers. Conventionally, it has been common to establish a threshold on the correlation levels involved in a delay profile in order to allocate the paths, no more than the number of the fingers, corresponding to the peaks that have levels in excess of the threshold to the fingers and drop the paths corresponding to the peaks that have levels below the threshold from the allocation to the fingers. In this approach, the cases may happen that the values of the correlation levels in the delay profile widely change under the environment in which, for example, multipath fading occurs, causing changeovers in the synchronization timing to be allocated to a certain finger and in turn causing frequent changeovers of the reception paths. Such frequent changeovers of the reception paths significantly deteriorate the reception characteristics of the CDMA reception system. Furthermore, a delay profile obtained through a single path search sometimes includes a high level noise. It has been proposed to perform an averaging process on the delay profile over a time period sufficiently longer than the fading period and make the path allocation through the use of the averaged delay profile thereby improving the accuracy of detecting the path as well as preventing occurrence of excessive changeovers of the paths. In this case, the threshold control is effected on the averaged delay profile to select the paths to be allocated to respective fingers. The averaged delay profile described above means the delay profile generated by taking a plurality of delay profiles as objects of an average calculation and averaging the values of the correlation levels at respective timings over the plurality of the delay profiles. In other words, in an averaged delay profile, each of the correlation levels is represented by the average of the correlation levels at respective specified timings in the plurality of signal-reception periods.
Since use of the averaged delay profile, however, cannot be promptly adaptive to the cases such as where a new intense path comes out and where a path that has so far existed is lost, a case could adversely occur that the quality of the reception signals is degraded. In order to be preventive of such situations, Japanese Patent Laid-open Application No. 2001-297076 (JP, P2001-292076A) describes a multipath detection circuit provided with both the section for averaging the delay profile over a long time period and the section for averaging the delay profile over a short time period to provide a path allocation to fingers on the basis of both of the averaging results. Japanese Patent Laid-open Application No. 2001-267958 (JP, P2001-267958A) describes an arrangement for adaptively controlling the number of the delay profiles employed for averaging.
Japanese Patent Laid-open Application No. 2000-115030 (JP, P2000-115030A) describes an arrangement in which, in selecting paths based on an averaged delay profile, weights are assigned to the currently selected paths to obviate occurrence of excessively frequent changeovers of the path.
FIG. 1 illustrates an example of a signal-reception device configured to detect the synchronization timing using the above-described prior art.
The CDMA receiver shown in FIG. 1 comprises antenna 1000 for receiving a radio signal; radio reception unit 1002 for converting radio signal 1001 received at antenna 1000 into baseband signal 1003; correlation calculation unit 1004 for calculating the correlation value of the baseband signal 1003 with the spreading code and providing delay profile 1005; delay-profile averaging unit 1006 for averaging the precedingly calculated delay profile and the presently calculated delay profile; precedingly-calculated delay-profile memory 1008 for storing an averaged delay profile calculated at delay-profile averaging unit 1006 and supplying the averaged delay profile to delay-profile averaging unit 1006 for the next calculation; and delay-profile calculation counter 1010 for updating the average calculation number (the number of calculations of averaging the delay profile) 1011 and supplying the updated averaging calculation number to delay-profile averaging unit 1006. Correlation calculation unit 1004 calculates the delay profile using, for example, a sliding correlator and a matched filter.
The CDMA receiver, furthermore, has path-level calculation unit 1012 for calculating path levels 1014-1 to 1014-N of the respective paths based on the averaged delay profile; path-timing calculation unit 1013 for calculating path timings 1015-1 to 1015-N of the respective paths based on the averaged delay profile; threshold processing unit 1016 for deciding whether or not path levels 1014-1 to 1014-N exceed the threshold; finger allocation unit 1019 adapted to receiving the information on path levels 1017-1 to 1017-K of K paths that exceed the threshold and also path timing 1018-1 to 1018-K corresponding to the K path levels, for allocating J paths of these K paths to fingers, J being a number within a predetermined number of the fingers; and demodulation processing unit 1021 for effecting despreading and demodulation processing on a reception signal 1003 making use of path timings 1020-1 to 1020-J of the paths allocated to the fingers as synchronization timings. Demodulation processing unit 1021 is provided with a predetermined number of fingers, each of which has a path timing established as a synchronization timing and despreads (demodulates) a baseband signal using spreading code based on the synchronization timing established on the finger of interest. Demodulation processing unit 1021 further RAKE-combines the despread signals provided from the respective fingers.
In the CDMA receiver, a precedingly calculated delay profile 1009 is stored in precedingly-calculated delay-profile memory 1008, and delay profile 1005 provided from correlation calculation unit 1004 and precedingly calculated delay profile 1009 are averaged by delay-profile averaging unit 1006. Then, delay-profile calculation counter 1010 updates average calculation number 1011 and supplies the updated result to delay-profile averaging unit 1006. Averaged delay profile 1007 is distributed to path-level calculation unit 1012 and path-timing calculation unit 1013. Path-level calculation unit 1012 selects N peaks of averaged delay profile 1007 in descending order of levels, where N is a natural number, and sends the levels of the selected peaks to threshold processing unit 1016 as N path levels 1014-1 to 1014-N. Path-timing calculation unit 1013 selects N peaks of averaged delay profile 1007 in descending order of levels, and sends the timings of the selected peaks to threshold processing unit 1016 as N path timings 1015-1 to 1015-N.
Threshold processing unit 1016 decides whether or not path levels 1014-1 to 1014-N for N paths exceed the threshold, and supplies path levels 1017-1 to 1017-K and path timing 1018-1 to 1018-K of the K paths that have path levels in excess of the threshold to finger allocation unit 1019. Finger allocation unit 1019 allocates to the fingers J paths selected out of K paths in descending order of path levels 1017-1 to 1017-K, J being a predetermined number, and provides path timings 1020-1 to 1020-J to demodulation processing unit 1021. In this case, if K>J, then some of path timings 1018-1 to 1018-K are dropped from the allocation to the fingers. If J is smaller than the number of the fingers, then there can be at least one finger to which a path is not allocated at this time point by finger allocation unit 1019.
Demodulation processing unit 1021 performs the demodulation processing such as the RAKE combination of the reception signals 1003 using path timings 1020-1 to 1020-J as synchronization timings and delivers the demodulation result. Before notified of the path timings from finger allocation unit 1019, demodulation processing unit 1021 holds the path timings notified for the last time and performs the demodulation processing using the holding path timings as synchronization timings. Thus, when the average processing of the delay profile is being implemented, demodulation processing unit 1021 performs the demodulation using the path timings calculated from the last averaged delay profile.
In the conventional CDMA receiver, since the path detection is performed on the basis of an averaged delay profile as described above, it has been possible to improve an S/N ratio and also to attain an improvement in an accuracy of the path detection through smoothing a noise level. However, since the path timing cannot be updated during the process of averaging the delay profile, it has been impossible to follow the timing variations, which have caused deterioration of the characteristics. This is because the averaging is effected not only with respect to the path levels but also with respect to time. Explanation is next given regarding the problem caused by the averaging made with respect to time.
Let a certain path (a propagation route) be focused on. Then, particularly if at least one of the sending end and receiving end moves, the movement will cause the path timing to change little by little. Consequently, the path timings involved in the latest delay profile of a plurality of delay profiles that have been subjected to averaging process will nearest approximate the current optimum path timings. The path timings involved in the averaged delay profile are under the influence of the path timings involved in delay profiles other than the latest delay profile, and it is presumed that the path timings derived from the averaged delay profile could deviate from the present optimum path timings.
In the CDMA communication system, the unit representative of the time length corresponding to the time duration for one bit of the spreading code is called “chip” and even only one chip of the time deviation in the synchronization timing causes failure in a normal despread. For example, only 0.7 chips of out-of-synchronization causes a significant deterioration in the reception characteristics (e.g., the reception sensitivity, the S/N ratio). It is not necessarily an optimum approach from the view of exact accordance of the synchronization timings to extract the path timings from the averaged delay profile.
FIGS. 2A to 2D exemplify the above-described matter, illustrating an add-and-average process of a power delay profile. While a delay profile is produced every signal-reception period (for example, 10 milliseconds), the time that is selected from and representative of the time period corresponding to the delay profile of interest is called a reception time in the present example. In the illustrated figures, path 1 at the timing of 10.0 chips and path 2 at the timing of 22.0 chips are detected from delay profile 1 at reception time τ1, as is shown in FIG. 2A; path 3 at the timing of 6.0 chips and path 4 at the timing of 10.5 chips are detected from delay profile 2 at reception time τ2, as is shown in FIG. 2B; and path 5 at the timing of 11.0 chips is detected from delay profile 3 at reception time τ3, as is shown in FIG. 2C.
In this example, if the paths originated from the primary reception signal are path 1, path 4 and path 5, then path 2 and path 3 are decided to be erroneous detections of noises as paths. Furthermore, it is known that the path created by the reception signal varies in the timing from 10.0 chips to 11.0 chips while the reception time elapses from τ1 to τ3. Making an add-and-average calculation of delay profiles 1, 2 and 3 yields an averaged delay profile as shown in FIG. 2D, in which path 6 is detected at the timing of 10.5 chips as a result of combination of path 1, path 4 and path 5 of the reception signal and the levels of path 2 and path 3 are smoothed and lowered below the threshold so as not to be detected. Thus, add-and-averaging of delay profiles improves the detection accuracy of the path by the advantage of smoothing noise levels.
In this example, it would be optimal to employ 11 chips as a synchronization timing for despreading the reception signal to demodulate it, because the timing of the path of the reception signal involved in the latest delay profile exhibits 11.0 chips. However, the timing of 10.5 chips is detected from the averaged delay profiles. As a result, effecting the despread through the use of the timing of 10.5 chips causes degradation of the reception characteristics such as a deterioration of the reception sensitivity. Thus, sole averaging of delay profiles involves also averaging with respect of the path timing, thereby deteriorating the follow-up characteristics to the variations of timings.
In addition, as an example of a circuit that is capable of executing a threshold process, the circuits adapted to effect the threshold processes on the averaged delay profiles to determine the paths to be allocated to fingers are described in Japanese Patent Laid-open Applications No. 2000-134215 (JP, P2000-134125A) and No. 2001-251215 (JP, P2001-251215A).
As described above, the conventional CDMA receivers are problematic in that effecting the path detection on the basis of an averaged delay profile makes it impossible to follow up the variation of the path timing causing the deterioration of the characteristics, while improving the accuracy of detecting the paths.